This invention relates to a solenoid valve, and more particularly to a solenoid valve wherein a push rod provided at a solenoid pushes a spool arranged in a valve body for the changing-over.
A conventional solenoid valve of such type is generally constructed as shown in FIG. 4.
More particularly, the conventional solenoid valve indicated at reference character Vu is adapted to change over a spool section 3 depending on an excitation current supplied to a solenoid 1 or 2. When the spool section 3 is at a neutral position shown in FIG. 4, a pump passage 4 and a tank passage 5 communicate with each other in the solenoid valve Vu.
When the solenoid 1 is excited, the spool section 3 is changed over from the neutral position to a left-side position to communicate a pump P with a left-side chamber 6 of a cylinder C and communicate a right-side chamber 7 of the cylinder C with a tank T. This causes a piston 8 of the cylinder C to be moved in a right-hand direction in FIG. 4. When the other solenoid 2 is excited, the spool section 3 is changed over to a right-side position, resulting in the piston 8 of the cylinder C being moved in a left-hand direction.
When the excitation current of each of the solenoids 1 and 2 is controlled, the stroke of the spool section 3 is controlled and a control pressure acting on the cylinder C is controlled depending on the stroke of the spool section 3.
Unfortunately, in the conventional solenoid valve constructed as described above, the piston 8 of the cylinder C is caused to be substantially moved even when the stroke of the spool is determined by controlling the excitation current of the solenoid, so that an increase in flow demand of the cylinder causes the control pressure to be decreased correspondingly. FIG. 5 shows a decrease of the control pressure with an increase in flow demand of the cylinder C from Q.sub.1 to Q.sub.3.
Thus, a decrease of the control pressure acting on the cylinder C causes the responsibility between the operation of the solenoid valve and the operation of the cylinder to be deteriorated.
Also, as shown in FIG. 6, a variation in temperature of oil causes the control pressure to be varied. More particularly, when the temperature of oil is lowered, the viscosity of oil is raised. However, such an increase in viscosity causes the stroke of operation of the cylinder C to be increased as compared with the stroke of the spool section 3. On the contrary, when the temperature of oil is raised, the viscosity is lowered; however, such lowering of the viscosity causes the stroke of operation of the cylinder to be decreased relative to the stroke of the spool section 3.
Thus, the conventional solenoid valve fails to keep the control pressure applied to the cylinder C constant even when the stroke of the spool section 3 is kept constant. This causes the responsibility to be unstable.
A valve for solving the above-described problem is proposed in U.S. Pat. No. 2,996,136, although it is not a solenoid valve. The proposed valve is used for a servo steering mechanism for a motor and includes a spool and two control pins slidably inserted in both sides of the spool. One end of each of the control pins is abutted against a valve body and the other end is associated with a reaction chamber formed in the spool. To the reaction chamber associated with one of the control pins is applied the loading pressure of a cylinder, and a reaction pressure varied depending on the velocity of a vehicle is applied to the reaction chamber associated with the other control pin.
Thus, in the proposed valve, the pressures applied to the two reaction chambers provided at both ends of the spool are different from each other, so that a load acting on the spool is biased. The application of such biased load to the spool causes the spool to carry out a twisting action at the contact region between the spool and a valve body to increase the friction therebetween. Such an increase in friction results in the responsibility of the valve being deteriorated.